Components of the above type are used to produce waves for pumping fiber optic amplifiers, such as those doped with erbium (EDFA).
For the above application in particular it is necessary to increase the gain of the semiconductor amplifiers and the optical power that they are capable of supplying.
FIG. 1 shows an example of a conventional semiconductor optical amplifier. It essentially comprises an active layer CA confined between a bottom buffer layer 2 and a top buffer layer 3. Biased bottom and top electrodes EB and EH inject a transverse pumping current through the active layer CA.
The region of the active layer within which the optical power is confined is called the amplifier waveguide (or the active waveguide). This region is defined by the vertical structure of the component. The structure can be of the graded index waveguide type. The active layer is then delimited laterally to define the width of the waveguide. The structure can instead be of the gain guided type. In this case the active layer is wider than the waveguide and the width of the waveguide is defined by the shape of the electrodes that localize the injected current, as in the example shown in FIG. 1.
Gain can be increased by lengthening the active waveguide of the amplifier, but this solution is limited by the phenomenon of the gain of the semiconductor amplifier medium saturating.
Gain depends on the length L of the active waveguide and on its gain coefficient g (also known as the "material gain") of the amplifier medium. The coefficient q is defined by:
dP/dz=g.P, where z is the position of a point along the longitudinal axis Oz of the amplifier medium and P is the optical power density at that point. PA1 G=.intg.g.dz, the integral being calculated along the longitudinal axis Oz between the input (z=0) and the output (z=L) of the waveguide. PA1 an input segment having an input waveguide adapted to guide a monomode input wave, PA1 a diffraction segment comprising a first medium transparent to said monomode input wave and adapted to widen it, PA1 a collimation segment, and PA1 an amplification segment having an amplifier waveguide wider than said input waveguide.
The gain G of the active waveguide (defined here as the ratio of the power densities at the output and at the input of the waveguide) therefore satisfies the equation:
The effect of the gain saturation phenomenon is that the material gain g decreases as the optical power density increases.
For a continuous wave, and to a first approximation: g=g0/(1+P/Ps), where g0 is the unsaturated gain coefficient (or peak value) and Ps is the saturation power of the medium. The coefficient g0 has a value that depends directly on the carrier density at the point concerned and therefore on the electric current density injected at that point.
Accordingly, for a given type of amplifier, increasing length L beyond a certain value leads to a very small relative increase in gain.
To mitigate that limitation, attempts were initially made to increase the saturation power Ps by optimizing the composition of the semiconductor layers constituting the amplifier, for example by adopting complex active structures such as multiple quantum wells.
Flared structure amplifiers have also been proposed in which the amplifier waveguide is flared in the direction of propagation of the amplified wave. The output power can be increased in this way without increasing the average optical power density in the waveguide. That solution increases the gain but is limited by the general need to retain a monomode output wave. What is more, the output wave from an amplifier of the above kind is highly astigmatic which makes it difficult to couple to an optical fiber.
Independently of the previous two approaches, attempts have also been made to maximize the unsaturated gain coefficient g0 by adjusting the pump current injected into the active layer.
Whichever solution is chosen, there remains another problem associated with the shape of the amplified wave. In the case of a monomode wave, for example, the amplitude of its electric field and therefore the optical power density vary along each lateral axis Oy of the waveguide with a maximum at the center of the waveguide. This field distribution is similar to that represented by a Gaussian curve. As shown diagrammatically in FIG. 2, from a particular amplification level the wave E0, E1, E2 then has an amplitude peak near the longitudinal axis Oz of the waveguide and this peak is amplified as it propagates. This reduces the efficiency with which the output wave is coupled to a monomode fiber.
In order to explain the above phenomenon, the reader should recall that a decrease in carrier density is accompanied by increased gain saturation. Since the injection of current is uniform throughout the active waveguide, the Gaussian shape of the wave implies that carrier density is lower at the center than towards the lateral edges, which should lead to a relative reduction of the amplitude at the center of the waveguide. However, the reduction in carrier density also causes an increase in refractive index, the effect of which is to confine the wave towards the longitudinal axis Oz of the waveguide, and it is this effect which predominates. There is therefore a phenomenon whereby the amplified wave is self-focused.